Sm-fe-N based alloy powder and process for producing the same

ABSTRACT

A magnetic powder of an Sm—Fe—N alloy, which has a mean particle diameter of 0.5 to 10 μm, and either an average acicularity of 75% or above or an average sphericity of 78% or above. The powder exhibits an extremely high residual magnetization and an extremely high coercive force, since particles characterized by the above acicularity or sphericity have particle diameters approximately equal to that of the single domain particle and nearly spherical particle shapes. The powder can be produced by preparing an Sm—Fe oxide by firing a coprecipitate corresponding to the oxide, mixing the obtained oxide with metallic calcium and subjecting the mixture to reduction/diffusion and nitriding successively.

This application is a division of application Ser. No. 09/582,293, filedJun. 23, 2000, now U.S. Pat. No. 6,334,908.

TECHNICAL FIELD

The present invention relates powder of alloy containing a rare earthelement Sm, a transition metal Fe, and nitrogen, and, more particularly,to such alloy powder having a spherical shape with superior magneticproperties.

BACKGROUND OF THE INVENTION

In recent years, there have been ever-increasing demands for rareearth-transition metal based magnetic materials because of theirsuperior magnetic properties, in spite of the fact that they are veryexpensive as compared with ferrite, etc. Among these, since Nd-basedmagnets have particularly high magnetic properties as compared withSm-based magnets, and are inexpensive, they have come to be mainly usedamong rare earth magnets.

Here, R—Fe—N based alloys have been known, which are formed by nitridingR—Fe based alloys that are rare earth-transition metal based magneticmaterials. Magnets of this type have been developed extensively sincethey have superior characteristics, including possibility of highercoercive force as compared with R—Fe—B based materials resulting fromhigh Curie Points not less than 150° C., high stability with smalltemperature variations in magnetic properties and high weatherresistance.

R—Fe—N based alloys are manufactured in the form of powder, molded intoa desired shape in combination with a filler, and then utilized asbonded magnets. Although alloy powder of this type exerts a highanisotropic magnetic field, a molded magnet made from the alloy powderof this type is difficult to obtain a high coercive force. In order toobtain a high coercive force, the magnet needs to be finely pulverized,or utilized as a metal bonded magnet containing a metal like Zn as abinder. In the case of the finely pulverized magnet alloy, the particlesare oxidized, or subjected to distortion or residual stress, resultingin degradation in other magnetic properties, e.g., a reduction inresidual magnetization. In the case of the metal bonded magnet, thismethod is considerably expensive as compared with plastic binding usedin normal bonded magnets, impossible to provide practical applications.

Magnetic alloy powder has an inherent mono-magnetic domain size, and ithas been known that magnetic powder whose particle size is set closer tothis mono-magnetic domain size can show a maximum coercive force. Formagnetic materials containing a rare earth element and a transitionmetal, the mono-magnetic domain size is several micrometers. Therefore,to improve magnetic properties of alloy powder serving as a magneticmaterial, it is essential to provide a process for forming fineparticles.

With respect to the process for a magnetic material containing a rareearth element and a transition metal, a reduction-diffusion method hasbeen known in which a mixture of powders of a rare earth metal oxide anda transition metal with metal calcium is heated in an inert gasatmosphere so that the rare earth oxide is reduced to metal andtransferred into the transition metal, to form an alloy (see JapanesePatent Publication Nos. JP-A61-295308, JP-A5-148517, JP-A5279714 and No.JP-A6-81010). This reduction-diffusion method is advantageous in that aninexpensive rare earth oxide may be used and alloued simultaneously withthe reducing process. This method has been widely used in manufacturingan intermetallic compound SmCo₅ or an Sm—Co alloy used for permanentmagnets. Moreover, in the case where the abovementioned R—Fe—N basedalloy powder is manufactured, after the R—Fe alloy has been reduced bythis method, the reduced alloy is subjected to a nitrding process toform magnetic powder of an R—Fe—N based alloy.

In this reduction-diffusion method, an oxide of a rare earth elementhaving a particle size of not more than several micrometers is used as amaterial, and the particle size of the magnetic powder obtained afterreduction becomes smaller to a certain degree; however, this method isstill not sufficient to provide fine magnetic powder corresponding tothe mono-magnetic domain size. This is because the particle size of amaterial iron-based metal is quite large as compared with that of therare earth element oxide. Therefore, conventionally, this reduced powderis nitrided, and then finely pulverized to the mono-magnetic domain sizeso as to exert a sufficient coercive force; thereafter, formed into abonded magnet, whereas the resulting bonded magnet exhibits only a lowresidual magnetization.

For bonded magnets, when its magnet particles are provided as fineparticles, its filling rate becomes low, resulting in a limitation inthe density of the magnetic powder contained in its molded body.Moreover, when the bonded magnet is oriented toward a magnetic fieldapplied, the distorted shape of the fine particles after pulverizationmakes it difficult to align the fine particles in a direction of easymagnetization axis toward the magnetic field, resulting in degradationin degree of alignment and degree of orientation.

DISCLOSURE OF THE INVENTION

An objective of the present invention is to provide powder of an Sm—Fe—Nbased alloy having high magnetic performances, in particular, a highcoercive force by optimizing a particle size and shape of the alloypowder.

Another objective of the present invention is to provide a method formanufacturing powder of an Sm—Fe—N based alloy having high magneticperformances, in particular, a high coercive force, without the need fora mechanical method such as a finely pulverizing process.

In the present invention, Sm—Fe—N based alloy particles are finelydivided to approximate a particle size to its mono-magnetic domain sizeor the vicinity thereof, and are simultaneously provided with aspherical shape, so that, when magnetizing bonded magnet toward amagnetic field, the fine particles can increase in degree of orientationin a direction of its easy magnetization, thereby increasing in coerciveforce.

In particular, in the Sm—Fe—N based magnetic powder of the presentinvention, the alloy powder is set to have an average particle size inthe range of 0.5 to 10 μm. Moreover, the magnetic powder is set to havean average degree of needle shape of not less than 75% in approximationof the spherical particles. In the present description, an averagedegree of needle shape is provided as an average of the degrees ofneedle shape of the individual particles which is defined by thefollowing equation:

Degree of needle shape=(b/a)×100(%)

where a represents the longest diameter on a projection image of aparticle, and b represents the largest diameter vertical to the a of theparticle. In particular, a shows the longest length on a particle imageprojected on a plane and b is the largest size vertical to the a on thesame projection.

In addition to the average particle size in the range of 0.5 to 10 μm,the Sm—Fe—N based magnetic powder of the present invention is set tohave an average degree of roundness of not less than 78% as means forestimating the spherical particles. Here, the average degree ofroundness is obtained as an average of values of roundness of therespective particles defined by the following equation:

Degree of roundness=(4nS/L)×100(%)

Here, S and L represent an area of a particle projection and aperipheral length of the outline of the particle image, respectively,which are measured on the particle image projected on the plane.

The process for producing Sm—Fe—N based magnetic powder of the presentinvention uses a combination of reduction-diffusion technique of a metaloxide and nitriding technique, in which not less than half or all of theFe source of a starting material for an Sm—Fe based alloy is prepared asiron oxide, and a mixture of the iron oxide with samarium oxide isreduced by a metallic reducing agent such as Ca. Thus, alloy particleshaving a shape distribution close to spherical shape are obtained. Themagnetic powder obtained by nitriding the alloy particles has aspherical shape or a similar shape to a spherical shape, allowing theparticles to easily rotate in a magnetic field direction when they aremagnetized in a resin bond. In this manner, the frequency of orientationof each magnetic particle toward the applied magnetic field is increasedso that the magnetic particles in a bonded magnet are easily magnetized.

In the present invention, from the fact that in the reduction-diffusionmethod, the size of reduced particles is greatly dependent on theparticle size of the material particles, oxide powder having fineparticles may be used as starting material particles.

For this purpose, the present invention may preferably use a mixture ofoxide particles of iron oxide and samarium oxide obtained through aco-precipitation method as a starting material. In other words, aprecipitation of the mixture of iron oxide and samarium oxide isobtained by the co-precipitation method from a solution in the presentof Fe and Sm co-existing, and decomposed and oxidized throughcalcination or another method to produce an oxide, and this oxide isavailable. In the present invention, the co-precipitation method andcalcination achieve a high degree of a mixed state between Fe and Sm,and provide very fine oxide particles having a spherical shape; thus,the resulting magnetic powder is reduced and diffused which have a sizeand a degree of needle shape similar to those of the material oxideparticles.

The method of the present invention further may include a process inwhich the oxide from the co-precipitation method is partially reducedpreliminarily, so that the material powder, preliminarily reduced, canbe more easily reduced and diffused by the metallic reducing agent suchas Ca as described above. In the preliminary reduction, a gas reducingprocess using hydrogen, etc. may be used, and the resulting mixture,part of the oxide of which has been reduced, contains metal iron, ironoxide and samarium oxide, and is used for reduction and diffusion.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the following attached drawings, the present invention willbe discussed in detail.

FIG. 1 is an explanatory drawing that schematically shows a method forcalculating a degree of needle shape from a particle projection image.

FIG. 2 is an explanatory drawing that schematically shows a method forcalculating a degree of roundness of a particle from a particleprojection image.

FIG. 3 is a graph that shows the relationship between coercive force andaverage particle size of powder of Sm₂Fe₁₇N₃ alloy.

FIG. 4 is a graph that shows the relationship between residualmagnetization and average degree of needle shape of powder of Sm₂Fe₁₇N₃alloy.

FIG. 5 is a graph that shows the relationship between coercive force andaverage degree of needle shape of powder of Sm₂Fe₁₇N₃ alloy.

FIG. 6 is a graph that shows the relationship between residualmagnetization and average degree of roundness of powder of Sm₂Fe₁₇N₃alloy.

FIG. 7 is a graph that shows the relationship between coercive force andaverage degree of roundness of powder of Sm₂Fe₁₇N₃ alloy.

BEST MODES FOR CARRYING OUT THE INVENTION

Alloy powder having a composition of Sm₂Fe₁₇N₃ is mainly used withrespect to Sm—Fe—N based magnetic powder of the present invention. Inparticular, the alloy powder includes a nitride having a compoundcomposition having 15 to 20 Fe atoms and 1 to 4 N atoms with respect to2 Sm atoms.

Alloy powder having an average particle size in the range of 0.5 to 10μm may be used as the Sm—Fe—N based magnetic powder of the presentinvention. In particular, the range of the average particle size maypreferably be set from 0.5 μm to 4 μm, more preferably from 0.6 μm to3.5 μm to increase the coercive force, and most preferably set from 0.7μm to 3 μm, so as to further increase the coercive force.

In the Sm—Fe—N based magnetic powder having the composition ofSm₂Fe₁₇N₃, FIG. 3 shows the relationship between average particle size(Da) of magnetic powder whose particle shape is substantially spherical(having not less than 95% in average degree of needle shape) and thecoercive force of a bonded magnet formed by mixing such powder particleswith a resin. The average particle size of the alloy particles shownhere is determined as follows. First, the specific surface area of thepowder is measured by using a Fischer subsieve sizer* through an airpermeation method, and from the results of this, the average value ofthe particle size of the primary particles is found to be used as theparticle size of the alloy particles. Magnetic measurements are carriedout on bonded magnets formed from a number of samples having differentparticle sizes, and data on the average particle size and the averagecoercive force are plotted on FIG. 3.

FIG. 3 shows that Sm₂Fe₁₇N₃ based particles have the highest coerciveforce is in the vicinity of 1 to 2 μm of the average particle size, andin this example, the value is 19 kOe. The reason that this particle-sizerange provides the highest coercive force is because this particle sizesubstantially approximates the mono-magnetic domain size of this alloy.

As the average particle size becomes smaller than this range, or greaterthan this range, the coercive force drops abruptly. In the case of anaverage particle size in the vicinity of 0.7 μm, the coercive force is17 kOe; in the vicinity of 0.6 μm, the coercive force is 15 kOe; and inthe vicinity of 0.5 μm, the coercive force is 10 kOe, approximately. Incontrast, in the average particle size of 4 μm, the value isapproximately 10 kOe. A increase in average particle size as large as 10μm decreases the coercive force to not more than 1 kOe.

Therefore, to attain a coercive force suitable for practical use, therange of the particle size of magnet spherical particles is set in therange of 0.5 μm to 4 μm in which a coercive force higher thanapproximately 10 koe is obtained, more preferably, in the range of 0.6μm to 3.5 μm in which a coercive force higher than approximately 15 kOeis obtained, and most preferably, in the range of 0.7 μm to 3 μm inwhich a coercive force higher than approximately 17 kOe is obtained.

Moreover, in the present invention, the magnetic powder is set to havean average degree of needle shape of not less than 75% in itsapproximation of the spherical particles.

In the present specification, the average degree of needle shape isprovided as an average value of the degrees of needle shape of therespective particles that is defined as follows: In the presentinvention, the degree of needle shape is found from the followingrelationship with respect to the respective particles.

Degree of needle shape=(b/a)×100(%)

where a represents the longest diameter on a projection image of aparticle, and b represents the largest diameter vertical to the a of theparticle. In particular, a shows the longest length on a particle imageprojected on a plane and b is the largest size vertical to the a on thesame projection.

The average degree of needle shape thus measured represents how close orhow different the average particle shape is to or from the sphericalshape.

The average degree of needle shape can be measured quickly at low costsby using graphical analyzing techniques using a computer. The averagedegree of needle shape may preferably be measured by the followingmethod. First, prior to measurements, measuring samples are provided ina manner so as to thinly spread alloy particles. These samples arespread as thinly as possible so as not to allow the particles to overlapeach other. A particle image is photographed by using an scanningelectron microscope (SEM) having a magnification of 4000 times, and theresulting data of the particle image is inputted to the computer througha scanner; thus, images showing shapes of respective particles areextracted, and image data of 100 particles are selected. Then, withrespect to each of the particle images, the longest length a and thelargest size b that is vertical to the longest length a are obtained bythe computer. FIG. 1 exemplifies measurements of the longest length aand the largest size b that is vertical thereto of an image of a certainparticle 1. In accordance with the above equation, the degree of needleshape of each particle is calculated, and an average of 100 particles isobtained to provide the average degree of needle shape. The averagedegree of needle shape thus measured indicates that the closer the valueto 100%, the closer the shape to the spherical shape.

In FIG. 4, for Sm₂Fe₁₇N₃ based particles, a number of data of alloypowders that have various average degrees of needle shape and also havean average particle size of approximately 1.5 μm are plotted; thus, therelationship between residual magnetization of bonded magnets andaverage degree of needle shape is shown. However, this Figure shows itstypical data, and it does not show a width of dispersion to a certaindegree which would exist in an actual case.

This figure shows that in the case of an average degree of needle shapeof not more than 70%, the residual magnetization is 87 emu/g, which issubstantially constant, and that the average degree of needle shapeexceeding 75% improves the residual magnetization to 89 emu/g. Moreover,in the average degree of needle shape of 80%, the residual magnetizationis 102 emu/g, in the average degree of needle shape of 85%, the residualmagnetization 125 emu/g, in the average degree of needle shape of 90%,the residual magnetization 133 emu/g, and in the average degree ofneedle shape of 95%, the residual magnetization 138 emu/g. In thismanner, as the magnetic particles become closer to the spherical shape,the residual magnetization is improved greatly. The results show thatwhen it is not less than 75%, the effect of an increase in the averagedegree of needle shape comes to be exerted on an increase in theresidual magnetization; therefore, the average degree of needle shape ispreferably set at not less than 80%, more preferably, not less than 85%,and most preferably, not less than 90%.

FIG. 5 shows the relationship between the coercive force and the averagedegree of needle shape based upon a number of data with respect to alloypowders that have average degrees of needle shape of various levels, andalso have an average particle size of 1.5 μm. This Figure shows typicaldata, and in actual cases, there is a certain degree of width. In thecase of an average degree of needle shape of not more than 70%, thecoercive force is not more than 5.8 kOe; however, in the case of anaverage degree of needle shape exceeding 75%, the coercive force isimproved to 8.2 kOe. When the average degree of needle shape is 80%, thecoercive force is 12.8 kOe, when 85%, it is 15.2 kOe, when 90%, it is17.3 kOe, and when 95%, it is 18.6 kOe; thus, as the particles becomecloser to the spherical shape, the coercive force is improved greatly.The results show that when it is not less than 75%, the effect of anincrease in the average degree of needle shape comes to be exerted on anincrease in the coercive force; therefore, the average degree of needleshape is preferably set at not less than 80%, more preferably, not lessthan 85%, and most preferably, not less than 90%.

Moreover, in the present invention, the particles of the magnetic powderare provided as Sm—Fe—N based alloy powder, and the average particlesize of the alloy powder is set in the range of 0.5 to 10 μm, and has anaverage degree of roundness of not less than 78%, which is representedby the average value of the number of particles obtained by thefollowing equation:

Degree of roundness=(4πS/L2)×100(%)

Here, S represents an area of particle projection on a plane, and Lrepresents a peripheral length of the outline of the same particle imageprojected on the plane.

The average degree of roundness is calculated in the following manner:First, with respect to measuring samples, a particle image isphotographed by using an SEM having a magnification of 4000 times, andthe resulting data of the particle image is inputted to the computerthrough a scanner; thus, images showing shapes of respective particlesare separated and extracted, and particle image data of 100 particlesare picked up. Then, with respect to each of the particle images, thearea S and the peripheral length L of the particle image are found bycomputer image analyses. As illustrated in FIG. 2, the area S and theperipheral length L of the particle image are found from a projectedimage of an elongated particle 1, and the value (4πS/L²)×100 of theabove-mentioned equation is found. In accordance with theabove-mentioned equation, for example, with respect to 100 particles,the degree of roundness is calculated for each of the particles, and anaverage of the 100 particles is calculated; thus, the average degree ofroundness is obtained. With respect to the degree of roundness, thecloser the value to 100%, the closer its cross-section to the roundshape.

FIG. 6 is a graph with plotted data that show the relationship betweenthe residual magnetization and the degree of roundness based upon anumber of data with respect to alloy powders that have average degreesof roundness of various levels, in the average particle size of 1.5 μm.This Figure shows typical data, and in actual use, there is a certaindegree of width. In the case of an average degree of roundness of notmore than 70%, the residual magnetization is not more than 87 emu/g;however, in the case of an average degree of roundness exceeding 78%,the residual magnetization is improved to 89 emu/g. When the averagedegree of roundness is 80%, the residual magnetization is 94 emu/g, whenthe average degree of roundness is 85%, it is 115 emu/g, and when theaverage degree of roundness is not less than 90%, it is 140 emu/g; thus,as the particles become closer to the spherical shape, the residualmagnetization is improved greatly. The results show that when it is notless than 78%, the effect of an increase in the average degree ofroundness comes to be exerted on an increase in the residualmagnetization; therefore, the average degree of roundness is preferablyset at not less than 80%, more preferably, not less than 85%, and mostpreferably, not less than 90%.

In FIG. 7, for Sm₂Fe₁₇N₃ based particles, a number of data of alloypowders that have average degrees of roundness in various levels andalso have an average particle size of 1.5 μm are plotted, showing therelationship between coercive force and average degree of roundness isshown. However, this figure shows typical data, and in actual use thereis a certain degree of width. With respect to the coercive force, thisFigure shows that in the average degree roundness of not more than 70%,the coercive force is 5.8 kOe, which is constant; however, in theaverage degree of roundness exceeding 78%, the coercive force isimproved to 8.2 kOe. When the average degree of needle shape is 80%, thecoercive force is 10.8 kOe, when 85%, it is 15.5 kOe, when 90%, it is18.4 kOe, and when 95%, it is 20.0 kOe; thus, as the particles becomecloser to the spherical shape, the coercive force is improved greatly.

The results show that when it is not less than 78%, the effect of anincrease in the average degree of roundness comes to be exerted on anincrease in the coercive force; therefore, the average degree ofroundness may preferably be set at not less than 80%, more preferably,not less than 85%, and most preferably, not less than 90%.

Here, the average degree of needle shape may be used for evaluating theshape of particles from macroscopic viewpoints, and can determiningwhether the particle projection view has a round shape or an ellipticalshape, or whether or not the particles aggregate. A low average degreeof needle shape indicates that most of the particles have an ellipticalor elongated shape with a narrowed portion. The elliptical shape of theparticles reduces a coercive force. Moreover, the particles cannot formmono-magnetic domain, resulting in a reduction also in the residualmagnetization. In particular, with not less than two particlesaggregating with each other cause reversed magnetic domains appear atthe neck portion.

Moreover, the particles not having a spherical shape, upon forming abonded magnet from the magnetic powder, raise problems of a low fillingrate and degradation in the magnetic field orientation.

The average degree of roundness is used for evaluating the shape ofparticles averaged from the microscopic viewpoint, and enables judgmentsas to whether or not there are protrusions, irregularity, and adhesionof fine particles on the surfaces of the particles. The protrusions andirregularity on the particle surface tend to cause a reversed magneticdomain therein, reducing coercive force. Moreover, the adhesion of fineparticles on the surface gives adverse effects on the formation ofmono-magnetic domain particles, then reducing residual magnetization. Inthe case of protrusions and irregularity on the particle surface, informing a bonded magnet of the magnetic powder, the particles collidewith each other, and subsequently tend to cause a stress on theparticle. This causes problems of a reduction in the coercive force anda failure in increasing the filling rate.

For this reason, in the present invention, the measurements of theaverage degree of needle shape and/or the average degree of roundnesscan represent a shape of particles by values from both of themacroscopic and microscopic viewpoints, and provides the relationshipbetween particle shape thus represented by figures and magneticproperties. Therefore, the measurements of the average degree of needleshape and/or the average degree of roundness on the can particle shapepredict the magnetic properties of a bonded magnet formed by a suchselection of particles.

Therefore, the magnetic powder of Sm₂Fe₁₇N₃ alloy of the presentinvention may be preferably provided as spherical particles having anaverage particle size in the range of 0.6 to 10 μm and an average degreeof needle shape of not less than 80%, obtaining a coercive force of notless than 12.5 kOe and a residual magnetization of not less than 100emu/g.

Moreover, the magnetic powder of Sm₂Fe₁₇N₃ alloy of the presentinvention may be provided as spherical particles having an averageparticle size in the range of 0.7 to 4 μm and an average degree ofneedle shape of not less than 85%; thus, it is possible to obtain acoercive force of not less than 15 kOe and a residual magnetization ofnot less than 125 emu/g.

With respect to the most preferable ranges, the magnetic powder of anSm₂Fe₁₇N₃ alloy of the present invention is provided as sphericalparticles having an average particle size in the range of 0.7 to 4 μmand an average degree of needle shape of not less than 90%; thus, it ispossible to obtain a coercive force of not less than 17 kOe and aresidual magnetization of not less than 130 emu/g.

As described above, the magnetic properties of the Sm₂Fe₁₇N₃ alloy aregreatly dependent on its average particle size, and shapes of theparticles. For methods for manufacturing the Sm—Fe—N based magneticpowder, Japanese Patent Publication JP-A6-151127 descloses powder ofSm—Fe based particles obtained through a method in which carbonyl ironpowder is used as material iron powder and the temperature of thereduction-diffusion method used in reducing the rare earth element isset in the range of 650 to 880° C. However, in this method, althoughhaving a certain degree of roundness, the particles are inferior inindependence and separation, and since those particles having a gourdshape with a narrowed portion or a twin joined shape are produced, onlyparticles having an average degree of needle shape of less than 70% areobtained. Moreover, since the particle size is greatly dependent on aparticle size of the carbonyl iron powder, the particle size are notsufficiently controlled. Consequently, it is inevitable to pulverize thepowder in order to obtain alloy powder that is capable of providingmono-magnetic domain size.

In an attempt to obtain powder having a spherical particle shape, it hasbeen conventionally known that alloy powder having a spherical particleshape can be obtained by gas-atomizing a melt having some components(for example, see Japanese Patent Publication JP-B7-50648). However, theaverage particle size is not less than 10 times as large as themono-magnetic domain size, and although no problems arise with respectto the filling rate and magnetic-field orientation, the particles are solarge as to have a multi-magnetic domain structure, with the result of aserious reduction in coercive force.

The following description will discuss a preferable process forobtaining the Sm₂Fe₁₇N₃ based alloy powder mentioned above having aspherical shape.

In the process for producing the alloy powder of the present invention,first, an oxide of Sm and Fe is used as a starting material, and theoxide of Sm and Fe is reduced and diffused with a metal reducing agent,and then subjected to a nitriding process. A co-precipitation method isadopted for preparing the oxide of Sm and Fe.

In the process utilizing the co-precipitation method and thereduction-diffusion method, the process may adopts steps of: dissolvingSm and Fe in an acid; reacting a substance in the aqueous solution,which substance produces a precipitate including an insoluble saltincluding Sm and Fe ions or a hydroxide, etc., so that an Sm compoundand an Fe compound are co-precipitated; calcining the resultingprecipitate to produce a metal oxide; and reducing the resulting metaloxide.

Moreover, in the production process of the present invention, theprecipitate of Sm and Fe is utilized, which is formed by precipitateparticles having a uniform distribution of constituent elements, a sharpparticle size distribution and a spherical particle shape, as a resultfrom a co-precipitation. The above method includes steps calcining theprecipitate to obtain a metal oxide and subsequently heating the metaloxide in a reducing atmosphere.

In this process, the steps for obtaining the precipitate particles isparticularly important because the shape of the precipitate particles,as it is, determines the particle size and shape of the metal oxideresulting from the oxidation thereof and the alloy powder resulting fromthe reduction thereof. Therefore, it is important to make the shape ofthe precipitate particles as close to the spherical shape as possible.

To obtain the alloy powder particles of the present invention, it ispreferable to set the precipitate particles to have a substantiallyspherical shape, and also to set the particle size and particle sizedistribution of the precipitate particles so as to be in the range of0.05 to 20 μm in average particle size and in the range of 0.1 to 20 μmin the total particle sizes.

Sm and Fe, which are positive ions in the constituent components, areuniformly mixed in water to obtain such precipitate particles. In orderto commonly dissolve these metal elements, preferable acids are mineralacids including hydrochloric acid, sulfuric acid and nitric acid, andthese can dissolve the above metallic ions at high concentrations.Moreover, chlorides, sulfates, or nitrates of Sm and Fe may be dissolvedin water.

Moreover, not limited to an aqueous solution, the solution may beprovided as a non-aqueous solution. For example, a solution formed bydissolving organic metals in the form of metal alkoxide in an organicsolvent such as alcohol, acetone, cyclohexane and tetrahydrofuran, maybe adopted.

A substance for producing insoluble salts with these ions is added tothe solution having the dissolved Sm and Fe ions. With respect to thissubstance, negative ions (nonmetallic ions), such as hydroxide ions,carbonate ions and oxalic acid ions, are preferably used. Any solutioncontaining a substance capable of supplying these ions may be used. Forexample, substances for supplying hydroxide ions include ammonia andsodium hydroxide. Substances for supplying carbonate ions includeammonium bicarbonate and sodium bicarbonate. Substances for supplyingoxalic acid ions include oxalic acid.

To the non-aqueous solution formed by dissolving the metal alkoxide inthe organic solvent is added water so as to allow the metal hydroxide todeposit. Besides these, any substance that reacts with a metal ion toproduce an insoluble salt may be applied to the present invention. Inparticular, a sol-gel method may be preferably used as the method forforming an insoluble hydroxide.

By controlling the reaction between metallic ions of two kinds andnon-metallic ions, it is possible to obtain an optimal alloy powdermaterial having a uniform metal-element distribution, a sharp particlesize distribution and a smooth spherical particle shape. The applicationof such a material improves the magnetic properties of alloy powder(magnetic material) that is a finished product. The control of thisprecipitation reaction can be carried out by properly setting factors,such as the supplying rate of the metallic ions and non-metallic ions,the reaction temperature, the concentration of the reaction solution,the stirring state of the reaction solution, and the pH at the time ofthe reaction. These conditions are first selected so as to optimize theyield of the precipitate, and then determined through observations undera microscope so as to maintain the unification (particle shape) of theprecipitate particles and a sharp particle size distribution of theprecipitate particles. Moreover, the state of the precipitate variesgreatly depending on what kinds of chemical substances are selected andwhat type of co-precipitation reaction is adopted. The conditions of theprecipitation process substantially determine the particle shape and theparticle size distribution of the alloy powder as the final magneticmaterial. As described earlier, the shape of the particles is closelyrelated to the magnetic properties of the magnetic material, and thecontrol of this precipitation reaction is therefore very important. Inthe precipitate particles obtained in this manner, Sm and Fe exist in asufficiently mixed state.

In the present invention, normally, it is preferable to remove thesolvent from the precipitate prior to the calcination. The sufficientremoval of the solvent in such a solvent-removing process makes thecalcination easier. Moreover, in the case where a solvent has such acharacteristic as to increase the solubility to a precipitate when thetemperature thereof increases, the precipitate particles partiallydissolve in the solvent, and a number of particles tend to aggregate,with the result that the central particle size increases and theparticle size distribution is widened; therefore, it is preferable tosufficiently remove the solvent.

At the time of the calcination process of the precipitate, the insolublesalt consisting of metallic ions and non-metallic ions is heated so thatthe non-metallic ions are decomposed, with the metal oxide being left.Therefore, it is preferable to carry out the calcination process in anoxygen-rich condition, that is, in an oxidizing atmosphere. Moreover,with respect to non-metallic ions, it is preferable to select those ionscontaining oxygen. Examples of such non-metallic ions include ions fromhydroxide, bicarbonic acid, oxalic acid, citric acid, etc.

In contrast, those ions such as sulfide ions allow these metals tocommonly deposit; however, since the oxidation of the sulfide is hardlycarried out unless the temperature is high, the application of thoseions is not preferable. Ions from phosphoric acid, boric acid andsilicic acid react with rare earth element ions or transition metal ionsto produce an insoluble salt; however, the resulting phosphate, borateand silicate are not easily allowed to form oxides in the followingcalcination process. Therefore, it is difficult to apply these to thepresent invention.

Therefore, in the present invention, besides ions from hydroxide,carbonic acid and oxalic acid, non-metallic ions, which are mostpreferably applied to the precipitation reaction, are those ions from aninsoluble organic salt that is easily burnt upon application of heat.

In the case where the insoluble organic salt is subjected to hydrolysisto produce a hydroxide, as in an alkoxide, it is preferable to once formit into a hydroxide and then to heat the hydroxide.

The purpose of this calcination is to decompose the nonmetallic ions soto obtain a metal oxide; therefore, the calcination is carried out at atemperature not less than a temperature at which such decomposingreaction takes 4place. Therefore, although the calcination temperaturevaries depending on the kinds of metallic ions and the kinds ofnonmetallic ions, it is preferable to calcine at a temperature in therange of 800 to 1300° C. for several hours, and more preferably, 900 to1100° C. In this case, the calcination is carried out in an air or in anoxidizing atmosphere. It is preferable to supply sufficient air to thecalcination furnace by using a ventilator, etc., or to introduce oxygeninto the furnace.

The calcination can provide a metal oxide having particles in which amicroscopic mixture of a rare earth element and a transition metalelement is made. These oxide particles are formed by an oxide havingsuperior particle performances that are derived from the shapedistribution of the above-mentioned precipitate particles.

In order to obtain alloy powder from the metal oxides, areduction-diffusion reaction is applied. In this case, the metal oxidesare Sm oxide and Fe oxide. Reduction of the iron oxide to metal isperformed by using H2, CO or hydrogen carbide such as CH4, while thereduction to iron is sufficiently carried out by introducing thesereducing gases into the furnace and heating it in a reducing atmospherethus formed. The reducing temperature may be set in the range of 300 to900° C. Temperatures below this range make it difficult to carry out thereduction of the transition metal oxide, and temperatures higher thanthis range cause the oxide particles to grow and to biasly deposit dueto the high temperature, resulting in deviations from a desired particlesize. The reducing temperature is preferably set in the range of 400 to800° C.

Moreover, in addition of the above-mentioned co-precipitation method,another method for sufficiently mixing the fine oxide particles of theconstituent elements may be adopted. More specifically, Sm₂O₃ having anaverage particle size of less than 5 μm and iron oxide having an averageparticle size of 2 μm are mixed, which later is once heated at atemperature in the range of 300 to 900° C. in a reducing gas so that theiron oxide is reduced to metal iron.

Although this method does not provide the homogeneity of mixture betweenSm and Fe so high as the co-precipitation method, it can carry out auniform mixing process better than the case in which metal iron is usedas the starting material, as conventionally made. The reason for this isexplained as follows:

(1) In the case where metal iron and samarium oxide are used as thestarting materials, an assumption is that metal iron powder having anaverage particle size of not more than 10 μm needs to be used, and thiscondition is satisfied by carbonyl iron powder. However, the carbonyliron powder industrially obtained has an average particle size ofapproximately 4 μm in the minimum. Here, in general, powder of Sm₂O₃ hasan average particle size around 1 μm, which is one-fourth of thecarbonyl iron powder ({fraction (1/64)} in volume). Under thesecircumstances, it is difficult to carry out a uniform mixing process.

(2) Carbonyl iron powder, which has an external appearance like a trueglobe and a smooth surface, is superior in fluidity, while Sm₂O₃ powderhas an irregular external appearance and is comparatively inferior influidity. Moreover, the bulk density of carbonyl iron powder is in therange of 3 to 4, while Sm₂O₃ is a comparatively small value of 0.9 indensity. Consequently, even if the two types of powder are mechanicallymixed with each other, it is not possible to obtain a satisfactoryresult.

(3) On the other hand, where both types of oxide powder are mixed witheach other, it is possible to industrially prepare materials maintainedin the same level in the particle size, fluidity and bulk density, andconsequently to provide a comparatively uniform mixture.

(4) In the present invention, a mixture formed by mixing oxides witheach other as the materials is temporarily reduced in a reducing gas sothat, in the same manner as the above-mentioned co-precipitation method,this may be subjected to a reduction-diffusion process using metal Ca orhydrogenated Ca.

The Sm oxide in the metal oxide cannot be reduced by a heating processunder the reducing gas atmosphere as mentioned above. The reducingprocess is available when a metal, which forms an oxide whose freeenergy of formation is smaller than the target Sm at the processingtemperature (in other words, the free energy of formation is negative,and its absolute value is greater), is mixed therewith and heated.Examples of such a reducing metal include alkaline metals such as Li,Na, K, Rb and Cs, and alkaline earth metals such as Mg, Ca, Sr and Ba.The rare earth metal oxide in the particles can be reduced to metal bymixing with such a metal having a great reducing strength and heating inan inert gas atmosphere. Here, from the viewpoint of security inhandling and costs, metal calcium or its hydride is most preferablyused.

In the reduction-diffusion method using an alkaline metal or an alkalineearth metal as a reducing agent, to a mixture of fine metal Fe powderobtained from a reduction by the reducing gas and a samarium oxide isadded metal calcium, or calcium hydride, and this is heated in an inertgas atmosphere or in a vacuum to allow the samarium oxide to contact thefused liquid or its vapor of the alkaline or alkaline earth metal and tobe reduced to metal samarium. With this reducing reaction, alloy powderof Sm and Fe can be obtained in a form of blocks.

Alkaline or alkaline earth metal reducing agent mentioned above is usedin the form of particles or powder; and in particular, from theviewpoint of costs, it is preferable to use granular metal calciumhaving a particle size of not more than 4 mesh. This reducing agent isused in an amount in the range of 1.1 to 3.0 times the reactionequivalent, and more preferably, 1.5 to 2.0 times the reactionequivalent (which is a stoichiometric amount required for reducing arare earth oxide, and in the case of use of a transition metal oxide,includes an amount required for reducing this).

It is possible to reduce iron oxide by using this reducing agent.Therefore, without the preliminary reducing process of the iron oxide bythe use of a reducing gas, the reducing process may be directly carriedout by using the Ca reducing agent. In this case, it is preferable topreliminarily reduce most of the iron oxide into metal prior to thereducing process of samarium by the use of the reduction-diffusionprocess. If all the predetermined amount of the iron oxide required forthe alloy composition is Ca-reduced, the amount of Ca required for thereduction becomes excessive, resulting in a high temperature due to theheat generation of Ca at the time of the reduction; this not only causesthe particle to become bulky, but also raises the possibility of theproduct being scattered in the furnace due to an explosive reaction inthe worst case. Therefore, the rate of removal of oxygen in the ironoxide prior to the reduction-diffusion process is preferably set at notless than 40% (a preliminary reduction rate to metal iron of not lessthan 40%). In the case of residual oxygen exceeding 60% in the ironoxide is be removed in the next Ca reduction process, a large amount ofthe Ca reducing agent is required, causing a problem of wasteful use, aswell as a failure in obtaining alloy powder particles having a uniformshape with superior dispersion. Here, the rate of removal of oxygen isreferred to as percentage of the amount of oxygen that is reduced andremoved with respect to the amount of oxygen as a whole that exists inthe oxide of the transition metal.

In the present invention, in addition to the reducing agent, a powderingagent may be used, if necessary. This powdering agent is used on demandso as to accelerate powdering of the product in the form of blocks anddispersion of the alloy particles upon carrying out a wet process, whichwill be described later. With respect to the powdering agent, examplesthereof include alkaline earth metal salts such as calcium chloride,which has been disclosed in Japanese Patent Publication JP-A 63-105909,and calcium oxide. Each of these powdering-accelerating agents is usedat a ratio in the range of 1 to 30% by weight with respect to the rareearth oxide used as the rare earth source, and more preferably, in therange of 5 to 30% by weight.

In the present invention, the above-mentioned material powder, reducingagent, and powdering agent, used if necessary, are mixed, and thismixture is loaded into a reaction furnace, and is subjected to asubstituting process in an inert gas atmosphere, such as argon gas,other than nitrogen, and heated so as to be reduced. Moreover, theheating temperature used in the reducing process is preferably set inthe range of 700 to 1200° C., and more preferably, 800 to 1100° C.Although the period of time of the heating process is not particularlylimited, it is normally set in the range of 10 minutes to 10 hours so asto carry out the reducing reaction uniformly, and more preferably, inthe range of 10 minutes to two hours. This reducing reaction provides aporous Sm—Fe based alloy in the form of blocks.

In order to obtain an Sm—Fe—N based alloy powder from the Sm—Fe basedalloy, the same Sm—Fe based alloy is successively subjected to anitriding process by introducing nitrogen gas into the same furnace.Instead of nitrogen gas, a compound gas, which can supply nitrogen afterhaving been decomposed by heat application, for example, ammonia, may beutilized. In the preceding reduction-diffusion process, the Sm—Fe alloyhas a porous granular shape; therefore, without being pulverized, thisis subjected to a heating process in the same furnace by immediatelyadjusting and switching to a nitrogen atmosphere. This operation makesit possible to subject the Sm—Fe alloy to a nitriding process uniformly,thereby providing an Sm—Fe—N alloy.

In this nitriding process, the temperature is lowered from the heatingtemperature range for the reduction to 300 to 600° C., and morepreferably, 400 to 550° C., and in this temperature range, theatmosphere is switched to the nitrogen atmosphere. The nitriding processtemperature less than 300° C. causes insufficient diffusion of nitrogeninto Sm—Fe based alloy that is a reaction product obtained in the aboveprocess, thereby making it difficult to effectively carry out thenitriding process uniformly. Moreover, the nitriding process temperatureexceeding 600° C. causes the Sm—Fe based alloy to be decomposed into arare earth nitride and metal iron a-Fe, resulting in serious degradationin the magnetic properties in the resulting alloy powder. Theabovementioned heating process time is set in such a range that thenitriding process is sufficiently carried out uniformly; and in general,the period of time is set in the range of 4 to 12 hours.

The reaction product of the nitriding process is a mixture containingbyproducts such as CaN, CaO and unreacted excessive calcium, as well asthe product alloy powder, and forms sintered blocks including these in acomposite form. Therefore, the product mixture is next put into coolwater so that CaN, CaO and metal calcium are separated from the alloypowder as Ca(OH)₂. Moreover, Ca(OH)₂ still remaining therein is removedby washing the alloy powder with acetic acid or hydrochloric acid. Inthe case where the Sm—Fe based alloy that is the porous granular productis put into water, oxidation of the metal calcium by water and ahydrating reaction of the byproduct CaO allow the sintered productmixture in the composite granular form to be powdered, that is, to beformed into fine powder.

The resulting slurry formed by the powdering is stirred, and hydroxidessuch as alkaline metal, etc., on the upper portion are removed throughdecantation, and processes of water pouring, stirring and decantationare repeated so that the resulting alloy powder is removed from thehydroxides. Moreover, partially residual hydroxides are completelyremoved by washing with acid in the range of pH3 to pH6, and morepreferably, pH4 to 5, such as acetic acid or hydrochloric acid. Afterthe completion of such a wet treatment, for example, after being washedwith water, this is further washed with an organic solvent such asalcohol or acetone, and then dehydrated and dried in a vacuum; thus,Sm—Fe alloy powder is produced.

EXAMPLES Example 1

1. Precipitation Reaction

Pure water (30 liters) was poured into a reaction tank, to this wasadded 520 g of H₂SO₄ of 97%, was dissolved 484.8 g of Sm₂O₃, and wasadded aqueous ammonia of 25% so as to adjust the pH to the vicinity ofneutral. To this aqueous solution was added 5200 g of FeSO₄·7H₂O andthis was completely dissolved to prepare a metal dissolved solution.

Pure water (12 liters) was poured into another reaction tank, and tothis were mixed 2524 g of ammonium bicarbonate and 1738 g of aqueousammonia of 25% to prepare an ammonium carbonate solution. The ammoniumcarbonate solution was gradually added to the metal dissolved solution,while the reaction tank was being stirred; thus, aqueous ammonia wasadded thereto so as to adjust the pH to 8.0±0.5 in the final stage afteraddition of all the amount thereof. The stirring was stopped withrespect to the solution, and this was allowed to stand still so that aproduct was precipitated on the bottom of the container. One portion ofthe precipitate thus obtained was sampled, and observed under amicroscope; thus, spherical particles with uniform grains were observed.The average particle size was measured to be 1.4 μm by using a Fischersubsieve sizer*.

2. Filtration and Washing

The precipitate product was washed on filter paper by being sucked whilesupplying ion exchange water from above. The decantation was continueduntil the electrical conductivity of the filtrate was reduced to notmore than 50 μS/m. The precipitate cake was washed, obtained by asuction filtering process, and dried in a dryer at 80° C.

3. Calcination in Air

The dried case was put into a crucible made of alumina, and calcined inthe air at 1100° C. for three hours.

4. Particle Size Adjustment

After the calcined matter had been disintegrated with hands, this wasfurther pulverized by a hammer mill. The particle size of this metaloxide powder was measured to be 1.3 μm by using a Fischer subsievesizer*.

5. Hydrogen Reduction

The pulverized powder was loaded into a tray made of steel, and this wasplaced into a tube furnace, and subjected to a heating treatment at 700°C. for 10 hours while hydrogen of a purity of 100% was allowed to flowat 20 liters/minute. The resulting black powder had an oxygenconcentration of 7.2 wt %.

6. Reduction-diffusion Reaction and Nitriding Reaction

The black powder (1000 g) obtained in the preceding process and granularCa (350.7 g) were mixed, loaded into a tray made of steel, and set in afurnace in an argon gas atmosphere. After the furnace had been evacuatedto a vacuum, this was heated at 1000° C. for one hour while argon gaswas being introduced thereto. Next, the heating process was stopped, andthis was successively cooled to 450° C. in the argon gas atmosphere;thereafter, this was maintained constant at this temperature. Then,after the furnace had been again evacuated to a vacuum, nitrogen gas wasintroduced thereto. This was heated for five hours while nitrogen gaswas being introduced at a pressure not less than the atmosphericpressure, and the heating process was then stopped, and cooled off.

7. Washing with Water

The resulting nitrided alloy powder was put into ion exchange water (5liters); thus, the reaction product was immediately allowed to powder tostart to separate into the alloy powder and Ca components. Processesincluding stirring in water, standing still and removal of supernatantwere repeated five times, and the resulting matter was finally washedwith 5 liters of an aqueous solution of acetic acid of 2 wt %; thus, theseparation of Ca components was completed. This was dried in a vacuum toobtain alloy powder of Sm₂Fe₁₇N₃.

8. Measurements on Magnetic Properties

The resulting powder had a superior dispersing property, andobservations under an electronic microscope showed that it had aspherical shape. The particle size of the powder was measured to be 2.5μm by using a Fischer subsieve sizer*, and the average degree of needleshape was 83%, while the average degree of roundness was 87%. The powderhad magnetic properties of σr 120 emu/g and iHc 15.8 kOe. Moreover, theconcentration of oxygen contained in the powder was 0.25 wt %, and nobias precipitation of Sm and Fe was observed under observations of thecross-section by means of EPMA. Moreover, no phases other than the mainphase of the Sm—Fe alloy were observed by an X-ray diffraction usingCu—Kα as the ray source, and in particular, no trace of α-Fe that was apure iron component was found.

Example 2

1. Co-precipitation

Samarium nitrate hexahydrate Sm(NO₃)₃·6H₂O (513.4 g), and iron nitratenonahydrate Fe(NO₃)₃·9H₂O (3432.3 g) were weighed, and weresimultaneously put into 10 liters of ion exchange water while beingstirred. After having been confirmed that they had been completelydissolved, to this was further added urea (NH₂)₂CO (2992.5 g) whilebeing continuously stirred, and the temperature of the solution wasraised to 80° C. while being continuously stirred. At this time, ureawas hydrolyzed into ammonia and carbon dioxide so that matter containingmetal was precipitated through a homogeneous reaction.

2. Filtration and Washing

The product was taken onto filter paper, and was sucked while supplyingion exchange water from the upper portion of the filter paper. Thisoperation was continued until the specific resistance of the filtratehad reached to not more than 50 pS/m. The washed cake was dried in adryer at 80° C.

3. Calcination in Air

The dried case was put into a crucible made of alumina, and calcined inthe air at 1100° C. for three hours.

4. Particle Size Adjustment

After the calcined matter had been disintegrated with hands, this wasfurther pulverized by a hammer mill. The particle size of this metaloxide powder was measured to be 1.3 μm by using a Fischer subsievesizer*.

5. Hydrogen Reduction; Preliminary Reduction

In order to preliminarily reduce iron oxide, the pulverized powder wasloaded into a tray made of steel, and this was placed into a tubefurnace, and subjected to a heating treatment at 700° C. for 10 hourswhile hydrogen of a purity of 100% was allowed to flow at 20liters/minute. The resulting powder obtained by this hydrogen reductionhad an oxyxgen concentration of 7.2 wt %.

6. Reduction-diffusion Reaction and Nitriding

Part of the powder (1000 g) obtained in the preceding process andgranular Ca (350.7 g) having a particle size of not more than 6 mm weremixed, loaded into a tray made of steel, and set in a furnace in aninert gas atmosphere. After the furnace had been evacuated to a vacuum,this was heated at 1000° C. for one hour while argon gas was beingintroduced thereto so that the mixture powder was reduced by calcium.Next, the heating process was stopped in the furnace, and this wassuccessively cooled to 450° C. in the argon gas atmosphere; thereafter,this was maintained constant at this temperature. Then, after thefurnace had been again evacuated to a vacuum, nitrogen gas wasintroduced thereto. This was heated for five hours while nitrogen gaswas being introduced at a pressure not less than the atmosphericpressure so that the reduced powder was nitrided, and the heatingprocess was then stopped, and the powder was cooled off in the furnace.

7. Washing with Water

The resulting product was put into ion exchange water (5 liters); thus,the reaction product was immediately allowed to powder to start toseparate into the alloy powder and calcium-containing components.Processes including stirring in water, standing still and removal ofsupernatant were repeated several times, and the resulting matter wasfinally washed with 5 liters of an aqueous solution of acetic acid of 2wt %; thus, the separation of Ca components was completed. This wasdried in a vacuum to obtain alloy powder of Sm₂Fe₁₇N₃

8. Measurements on Magnetic Properties The resulting powder had asuperior dispersing property, and observations under an electronicmicroscope showed that it had a spherical shape. The particle size ofthe powder was measured to be 2.8 μm by using a Fischer subsieve sizer*.The powder had magnetic properties of σr 140 emu/g and iHc 18 kOe. Theconcentration of oxygen contained in the powder was 0.25 wt %, and nobias precipitation of Sm and Fe was observed under observations of thecross-section by means of EPMA. Moreover, no phases other than the mainphase of the Sm—Fe alloy were observed by an X-ray diffraction usingCu—Kα as the ray source, and in particular, no trace of metal iron (α-Fephase) was found.

Example 3

Iron oxide (Fe₂O₃) powder (135.7 g) having an average particle size of1.5 μm and a purity of 99.9% and samarium oxide (Sm₂O₃) powder (34.9 g)having an average particle size of 1.0 μm and a purity of 99.9% werekneaded together with water in a ball mill for two hours. The iron oxidepowder and samarium oxide powder used here were the same as those usedin Example 1. The resulting slurry was dehydrated to separate a solidmatter, and the dried solid matter was pulverized in a sample mill toprovide mixed powder. The resulting mixed powder was put into a traymade of soft steel, and preliminarily reduced in a furnace at 600° C. inan hydrogen flow. During the reducing process, the flow rate of hydrogenwas set at 2 l/min., and the maintaining time was five hours.

The results of an oxygen analysis on the reduced powder showed that theoxygen removal rate of the iron oxide component was 89.5%. Under theseconditions, the samarium oxide was not reduced by hydrogen gas. To thereduced mixture powder (178 g) was added granular metal calcium (44.50g), and this was sufficiently mixed. This was loaded into a cruciblemade of soft steel, and then subjected to the same processes as those ofExample 1 so as to be reduced in an electric furnace; thus, Sm—Fe—Nalloy powder was obtained.

The resulting powder had a superior dispersing property, andobservations under an electronic microscope showed that a number ofparticles thereof had a spherical shape. The particle size of the powderwas measured to be 2.0 μm by using a Fischer subsieve sizer*. Theaverage degree of needle shape was 78%, and the average degree ofroundness was 81%. A bonded magnet manufactured from this powder hadmagnetic properties of σr 102 emu/g and iHc 12 kOe. The concentration ofoxygen contained in the powder was 0.15 wt %, and no bias precipitationof Sm and Fe was observed under observations of the cross-section bymeans of SEM. Moreover, no phases other than the main phase of the Sm—Fealloy were observed by an X-ray diffraction using Cu—Kα as the raysource, and in particular, no trace of metal iron (α-Fe phase) wasfound.

Comparative Example 1

For comparative purposes, magnetic powder was produced by using a fusingmethod. Metal Sm and metal iron were fused in an atomic ratio of 2 to17, and the fused matter was injected into a water-cooling copper moldso that an alloy having a composition of Sm₂Fe₁₇ was obtained. Theresulting ingot was coarsely pulverized by a jaw crusher, and the powderwas heated and maintained at 1100° C. in an argon gas for 40 hours so asto be homogenized.

The resulting alloy powder was pulverized for two hours in a ball millwith steel balls, and subjected to a heating process at 450° C. for fivehours in an atmosphere of nitrogen of 100%. The resulting powder wasinferior in the dispersing property and in an aggregated state, andobservations under an electronic microscope showed that the particleshad polygonal shapes. The average particle size of the powder was 10 μmbased upon measurements by using FSSS, the average degree of needleshape was 64%, and the average degree of roundness was 67%. With respectto the magnetic properties of the powder, the residual magnetization σrwas 85 emu/g and the coercive force iHc was 8.2 kOe. The concentrationof oxygen contained in the powder was 0.6 wt %, and bias precipitationof Sm and Fe was observed under observations of the cross-section bymeans of EPMA. Moreover, a clear peak due to α-Fe was observed by anX-ray diffraction using Cu—kα as the ray source.

As described above, since the alloy powder of the present invention hasa constant spherical particle shape, it is possible to greatly improvethe residual magnetization and the coercive force. The reason for thisis explained as follows: Since the fine powder close to themono-magnetic domain size is obtained without using a mechanical stresssuch as a pulverizing process, it is possible to reduce distortions,cracks, scratches, etc. on the surface that give serious adverse effectson the magnetic properties, and consequently to properly set themagnetic field orientation with ease so as to form spherical particles.

Moreover, when a molded member such as a bonded magnet is formed byusing the alloy particles of the present invention, it is possible toimprove the residual magnetization of the molded member. This isbecause, in the process for molding the member with the alloy particlesbeing aligned in a direction of its axis of easy magnetization withinthe magnetic field, the application of the alloy powder having sphericalparticles makes it possible to greatly improve the degree of alignment.

In the process of producing the present invention, a reduction-diffusionmethod such as calcium reduction is utilized by using material powderincluding an oxide; therefore, spherical magnetic powder can be directlyobtained, thereby making it possible to reduce distortions, cracks,scratches, etc. that give serious adverse effects on the magneticproperties. Thus, the resulting spherical particles make it possible toeasily set the magnetic field orientation in the bonding agent uponmagnetization. Consequently, it becomes possible to obtain magneticpowder that has improved magnetization and coercive force and issuitable for a bonded magnet.

In the process of the present invention, in particular, aco-precipitation method, which allows an insoluble salt or a hydroxideto precipitate in water, is utilized so as to obtain material particlesincluding iron and samarium; therefore, since the material particles arefine, and since the elements, which will constitute the alloy powder,are uniformly mixed at their material stages, the particles, which havebeen subjected to a reduction-diffusion process and a nitriding process,have a diameter close to the mono-magnetic domain size, without the needfor a mechanical stress such as a pulverizing process, and are allowedto have a spherical shape.

In the normal fusing method, that is, in the process of producing alloypowder in which ingots of samarium and iron are formed and thenpulverized, in most cases, a thermal process which lasts for as long asseveral tens of hours needs to be provided in order to obtain ahomogeneous alloy. In contrast, in the precipitation method, the thermaltreatment time only requires two hours at longest. Therefore, theshortened thermal treatment time makes it possible to easily providespherical particles.

Since the particle shape of the precipitate particles obtained in theprecipitation process is closely related to the alloy powder as thefinished product, it is possible to obtain alloy powder having a uniformparticle shape with superior dispersion by controlling the shape of theprecipitate particles, and consequently to provide a magnetic materialhaving superior magnetic performances.

Industrial Applicability

The Sm—Fe—N based alloy powder of the present invention can bemanufactured by magnet manufacturing makers, and in the magnetmanufacturing makers, the process is used for making such alloy powder,and the alloy powder is formed into a predetermined shape as a bondedmagnet so as to be applied to a permanent magnet used in various fieldssuch as electric appliances, information communication apparatuses andmachines.

What is claimed is:
 1. A process for producing a permanent magnetmaterial comprising Sm—Fe—N based alloy powder, comprising: allowing aprecipitate containing Sm and Fe to co-precipitate from a solution inwhich Sm and Fe are dissolved; calcining the precipitate to form a metaloxide; mixing the metal oxide with a metal reducing agent; reducing anddiffusing the metal oxide mixed with the metal reducing agent into Sm—Fealloy powder; and nitriding the Sm—Fe alloy powder to obtain saidSm—Fe—N based alloy powder, wherein said Sm—Fe—N based alloy powder hasa particle shape of an average degree of roundness of not less than 85%.2. The process for producing a permanent magnet material comprisingSm—Fe—N based alloy powder according to claim 1, wherein said Sm and Feare uniformly distributed in each particle.
 3. The process for producinga permanent magnet material comprising Sm—Fe—N based alloy powderaccording to claim 1, further comprising heating said metal oxide formedby calcining the precipitate at a temperature in a range from 300 to900° C. in a reducing gas to preliminarily reduce all or part of theiron oxide into metal iron before said reducing and diffusing theresulting metal oxide powder into Sm—Fe alloy powder.
 4. The process forproducing a permanent magnet material comprising Sm—Fe—N based alloypowder according to claim 1, wherein said Sm—Fe—N based alloy powder hasa particle shape of an average degree of roundness of not less than 90%.5. A process for producing a permanent magnet material comprisingSm—Fe—N based alloy powder, comprising: allowing a precipitatecontaining Sm and Fe to co-precipitate from a solution in which Sm andFe are dissolved; calcining the precipitate to form a metal oxide;mixing the metal oxide with a metal reducing agent; reducing anddiffusing the metal oxide mixed with the metal reducing agent into Sm—Fealloy powder; and nitriding the Sm—Fe alloy powder to obtain saidSm—Fe—N based alloy powder, wherein said Sm—Fe—N based alloy powder hasan average particle size of 0.6 to 10 μm and a particle shape having anaverage degree of needle shape of not less than 80%.
 6. The process forproducing a permanent magnet material comprising Sm—Fe—N based alloypowder according to claim 5, wherein said Sm—Fe—N based alloy powder hasan average particle size of 0.7 to 4 μm and a particle shape having anaverage degree of needle shape of not less than 85%.
 7. The process forproducing a permanent magnet material comprising Sm—Fe—N based alloypowder according to claim 5, wherein said Sm—Fe—N based alloy powder hasan average particle size of 0.7 to 4 μm and a particle shape having anaverage degree of needle shape of not less than 90%.
 8. A process forproducing a permanent magnet material comprising Sm—Fe—N based alloypowder, comprising: allowing a precipitate containing Sm and Fe toco-precipitate from a solution in which Sm and Fe are dissolved;calcining the precipitate to form a metal oxide; mixing the metal oxidewith a metal reducing agent; reducing and diffusing the metal oxidemixed with the metal reducing agent into Sm—Fe alloy powder; andnitriding the Sm—Fe alloy powder to obtain said Sm—Fe—N based alloypowder, wherein said Sm—Fe—N based alloy powder has an average particlesize of 0.6 to 10 μm, a particle shape having an average degree ofneedle shape of not less than 80%, a coercive force of not less than12.5 kOe and a residual magnetization of not less than 100 emu/g.
 9. Theprocess for producing a permanent magnet material comprising Sm—Fe—Nbased alloy powder according to claim 8, wherein said Sm—Fe—N basedalloy powder has an average particle size of 0.7 to 4 μm, a particleshape having an average degree of needle shape of not less than 85%, acoercive force of not less than 15 kOe and a residual magnetization ofnot less than 125 emu/g.
 10. The process for producing a permanentmagnet material comprising Sm—Fe—N based alloy powder according to claim8, wherein said Sm—Fe—N based alloy powder has an average particle sizeof 0.7 to 4 μm, a particle shape having an average degree of needleshape of not less than 90%, a coercive force of not less than 17 kOe anda residual magnetization of not less than 130 emu/g.
 11. The process forproducing a permanent magnet material comprising Sm—Fe—N based alloypowder according to claim 1, wherein the metal reducing agent ismetallic Ca or calcium hydride.